When it comes to developing a permanent defect, the most difficult task is to determine how to generate coercivity. This is also true of an R-T-B based sintered magnet. That is why researches and developments are still carried on to find out exactly how the coercivity is generated.
In practice, several methods for increasing the coercivity of an R-T-B based sintered magnet are known. One of those methods is using a heavy rare-earth element (such as Dy or Tb, among other things) as one of the rare-earth elements as disclosed in Patent Document No. 1. However, only a limited amount of Dy or Tb can be added because Dy and Tb are rare and expensive elements and because an excessive amount of Dy or Tb added would interfere with forming a main phase when a material alloy is prepared.
Meanwhile, to increase the coercivity, not just such rare-earth elements but also various other elements have been added tentatively as well. For instance, Al or Cu is usually added as disclosed in Patent Document Nos. 2 and 3, respectively. However, these elements are regarded as contributing effectively to improving the metallic structure of a magnet, rather than the magnetic properties of an R2T14B type compound that is a ferromagnetic phase. That is why even if a small amount of such an element is added, the coercivity would still increase. Among other things, Cu has the effect of relaxing considerably the conditions of heat treatment to be normally carried out on an R-T-B based sintered magnet after the sintering process. This is believed to be because Cu would be distributed in the form of a film over the interface between the main phase and the grain boundary phase and eliminate microscopic defect surrounding the main phase. If a lot of Cu were present, however, not only the remanence but also the coercivity would rather decrease. For that reason, only a limited amount of Cu can be added and the effect achieved by adding Cu has been marginal so far.